Human platelets demonstrate a wide variety of morphological shapes and sizes. Some believe that this represents a metamorphosis as the platelet passes from discoid shape to round shape to bizarre shape with pseudopod extensions. Others believe this represents a senescence of the platelet and that these forms differ in their potential functional ability to initiate thrombus formation. It is possible that the discoid shaped platelets may be associated with greater adherence, aggregation, and release properties than round or pseudopod shaped platelets. A new device to noninvasively determine the proportion of discoid-shaped platelets in platelet rich plasma has been developed and standardized. In vivo studies in animals and man involve isotope tagging of pure forms of the animal and human platelet shapes and measuring platelet deposition sites by single and dual radionuclide imaging. Additionally, half life survival times will be determined. This study may lead to a significant improvement in post-operative bleeding syndromes associated with cardiac surgery.